The present invention relates to a dope composition for application to the band ply or inner liner of a "green" vehicle tire before molding and to a process for the production of such dope composition.
In the production of vehicle tires it is conventional practice to mold the green tire in a molding press in which the tire is pressed outwardly against an outer mold, which defines the tread pattern and the shape of the side walls, by means of a bladder which is sequentially pressurized with low pressure steam, followed by high pressure steam or hot water, followed by cold water and then evacuated by draining the cold water and applying vacuum service. Other methods can be used to collapse the bladder.
One of the principal functions of the dope is to act as a lubricant between the tire inner liner and the press bladder both during the loading stage and the stripping stage of the molding operation; particularly during inflation of the bladder in the loading stage, there is a substantial relative movement between the contacting surfaces of the bladder and the tire inner liner. Unless adequate lubrication is provided between tire and liner there is a tendency for the bladder to buckle, which may result in mis-shaping of the tire and consequent rejection of the tire. At the final stage during bladder collapse and the stripping of the tire from the bladder, there is again considerable relative movement between the contacting surfaces of the bladder and the now cured, tire inner liner. Unless adequate lubrication is provided between bladder and liner excessive wear and roughening of the bladder will occur, resulting in reduced bladder life.
Another of the principal functions of the dope is to avoid entrapment of major air bubbles between the tire inner liner and the bladder at the beginning of the pressing operation and to promote entry of air between the bladder and the tire inner liner at the end of the molding operation to avoid adhesion of the tire inner liner to the bladder when the bladder is evacuated prior to withdrawal from within the tire. The entrapment of air between the bladder and the inner liner and failure of the liner to separate from the bladder on evacuation of the bladder at the end of the molding cycle may both lead to such severe defects in the molded tire as to require it to be rejected.
It has been the practice therefore to employ a band ply dope which incorporates a lubricant, dispersed or dissolved in a liquid medium, and fine solid particles, such as mica or graphite, in suspension in the liquid medium. While other solid particles may be employed, mica and graphite are preferred because of their laminar nature. It is conventional to employ these materials in finely divided form, but having a substantial proportion of particles of substantial size. For example, mica for this purpose is generally required to pass 100% through a 100 BSS mesh sieve (to be smaller than 150 microns) and to have a substantial proportion, say 20%, retained on a 325 BSS mesh sieve (to be larger than 44 microns).
It has heretofore been the general practice to apply a band ply dope by means of a brush to some types of tire. Since this is a very monotonous job, the applied dope coating is rarely uniform and it is quite possible for some areas to be omitted. Where there are areas of the inner liner which are uncoated with dope, particularly in the region of the beads, defects due to adhesion to the bladder can be very severe.
Troubles due to misapplication of the dope may be overcome by spraying the dope since this can lead to much more uniform and complete coatings. In fact, known alcohol-based and petroleum-based dopes have been applied by spraying, but all known alcohol-based and the majority of the petroleum-based dopes produced soft dope films, which are unsuitable for use with certain classes of tire, such as radial-cord tires. With such tires the contact movement between the bladder of the press and the inner liner of the green tire at the beginning of the molding cycle is sufficient to rub off a soft dope film, particularly in the bead region, where it is important to have an effective dope film.
The known alcohol-based and petroleum-based dopes, although they dry very rapidly, are very mobile and suffer from the disadvantage that they run after being sprayed and form local pools of dope, which, on drying, leave thick areas of dry dope which may flake off, leading to other difficulties. Furthermore, they suffer from the disadvantage that the suspended mica or like solid particles tend to separate from the liquid medium under the shear conditions existing at the spray nozzle, which lead to nozzle blockage and to improperly doped areas, which have been wetted with the liquid medium, but carry no applied solid. Dopes, which dry to a hard, tough film on the inner liner of the green tire, are known, but these have not been suitable for application by spraying.
It is an object of the present invention to provide a composition suitable for use as a band ply dope and which is both satisfactory for application by spraying and dries to a tough film, which provides both the lubrication properties and air admission properties required from a band ply dope.
There are various considerations to be taken into account in selecting the liquid medium, which acts as the continuous phase of the dope. It must first of all have sufficiently high volatility at room temperature to permit the dope film to dry within the normal time interval between the application of the dope and introduction into the press, say 30 minutes. The liquid medium must therefore exert a substantial vapor pressure at room temperature and thus should boil at a temperature not in excess of about 85.degree.-90.degree.C. and preferably somewhat below that level. The liquid medium must not present any health or excessive fire hazard and must not have any detrimental effect on the green tire through attack of the rubber.
As previously stated, the dope composition is required to be sprayable, which implies that the dispersed liquid and solid materials do not separate out at the nozzle under spraying conditions. It can be seen from the previous discussion of the difficulties which arise with the known compositions that it is very desirable that a sprayed film of the dope composition should not run and form pools on the surface of the band ply.